For the condenser microphone, an electrostatic acoustic-electric converter provided in a microphone unit has a very high impedance. Therefore, the microphone incorporates an electronic circuit including an impedance converter. As the impedance converter, a field effect transistor (FET) has usually been used.
Therefore, when a high-frequency current caused by external electromagnetic waves (especially, electromagnetic waves emitted from a cellular phone) is applied to the electronic circuit, the current is detected by the FET, and noise of an audible frequency may be generated.
This problem is solved by reliable shielding of a microphone housing. However, for the condenser microphone of a type such that a microphone capsule is exchangeable with respect to a microphone main body, since electrical connection is made in a mechanically connected portion, the high-frequency current is liable to intrude into the microphone housing through the connecting part.
The intrusion of the high-frequency current into the FET can be prevented by ferrite beads mounted at the gate of FET.
However, since the gate part of FET has a high impedance, vibrating noise may be generated if the ferrite beads vibrate mechanically, and leak noise may be generated if the electrical insulation is insufficient.
As a solution of this vibrating noise problem, the present applicant has proposed, in Japanese Patent Application Publication No. 2007-28027, a technique in which the extraction electrode of a backplate on the microphone capsule side and the gate of the FET on the microphone main body side are electrically connected to each other via a conductive elastic material such as a conductive cloth.
According to this technique, the conductive elastic material is in planar contact with the extraction electrode of backplate and the gate of FET with a large area. Therefore, the contact resistance value is reduced accordingly, and a stable connecting state can be formed, so that the vibrating noise can be reduced effectively.
Accordingly, an object of the present invention is to provide a condenser microphone configured so that a microphone capsule is exchangeable with respect to a microphone main body, in which electromagnetic shielding is made more reliable by providing a conductive elastic material such as a conductive cloth and a ferrite material in the connecting part of the microphone capsule and the microphone main body.